Stirling motor and heat pump

ABSTRACT

Stirling engine which may be used as a heat pump, which consists of a hot half and a cold half. Both halves are connected by two lines which constitute a counterflow heat exahnger or in which a counterflow heat exchanger is mounted. Moreover, a mutual shaft, to which in the hot half a large and a small piston are mounted and to which in the cold half a large and a small piston are mounted, connects both parts. For every up or down going movement of the shaft, a complete Stirling cycle is performed. If desired, the shaft may be replaced by a hydraulic interconnection.

The invention relates to a Stirling motor provided with at least onepiston, which is movable in a reciprocating manner in an operationallyhot motor part and a cold motor part. The Stirling motor as invented in1817 by Stirling, consists of a cylinder, which is heated on one sideand cooled on another side. In the cylinder a displacer and a piston canmove freely. The displacer and the piston are each individuallyconnected to a flywheel. In the Stirling motor a Stirling cycle isexecuted, during which work can be done by the piston.

The disadvantage of the known Stirling motor is that the heat and thecold must be brought substantially to one location, while in practice aheat source and a cold source are often available on differentlocations. The Stirling motor according to the invention substantiallyobviates this disadvantage and is characterized in that the motorcomprises a separate hot motor part and cold motor part, which areconnected by two tubes and a shaft or a hydraulic interconnection.

A favourable embodiment of the inventive Stirling motor is characterizedin that the hot motor part is provided with a first system of twomutually coupled pistons, that the cold motor part is provided with asecond system of two mutually coupled pistons and that the shaft or thehydraulic interconnection forms a connection between the first systemand the second system. In this manner, the entire isothermal expansioncan take place in the hot motor part and the entire isothermalcompression can take place in the cold motor part. An additionaladvantage is that in this way a Stirling motor is obtained whichperforms a complete and substantially continuous Stirling cycle forevery single stroke of the reciprocating pistons.

A further favourable embodiment of the inventive Stirling motor ischaracterized in that the two tubes are mutually thermallyinterconnected by a counterflow heat exchanger. Preferably the tubesthemselves are closely thermally connected across their entire length,such that they can be used for exchanging heat during the isochorouspart of the Stirling cycle.

A favourable embodiment according to another aspect of the invention ischaracterized in that the first system of coupled pistons comprises alarge and a small piston, which can move in a first assembly of a largeand a small cylinder and that the second system of coupled pistonscomprises a large and a small piston, which can move in a secondassembly of a large and a small cylinder. In this embodiment the ratiobetween the diameters is according to the invention at leastsubstantially determined by the temperature difference to be expectedbetween the heat source and the cold source.

A favourable embodiment according to another aspect of the invention ischaracterized in that the four cylinders are provided with eightconnections and that a system of valves is provided for mutuallyconnecting the eight connections for executing a Stirling cycle. In thisway a switchover can be made at the right moment, that means the mostoptimal moment from one part of the Stirling cycle to the next part.

The invention also relates to a heat pump provided with at least onepiston, which can be moved in a reciprocating manner in an operationallyhot pump part and a cold pump part. The inventive heat pump ischaracterized in that the heat pump consists of a separate hot pump partand cold pump part, which pump parts are connected by two tubes and ashaft or a hydraulic interconnection. It is possible then to locate thecold pump part for example in the soil and the heat pump part in ahouse, in such a manner that all produced heat can be utilised.

A favourable embodiment of the inventive heat pump is characterized inthat the hot pump part is provided with a first system of two mutuallycoupled pistons, that the cold pump part is provided with a secondsystem of two mutually coupled pistons and that the shaft or thehydraulic interconnection forms a connection between the first systemand the second system. In this way the isothermal compression may takeplace completely in the hot pump part and the isothermal expansioncompletely in the cold pump part. Moreover, in that way a heat pump isobtained which performs for every reciprocating stroke of the pistons acomplete and substantially continuous Stirling cycle.

A further favourable embodiment of the inventive heat pump ischaracterized in that the two tubes are mutually thermallyinterconnected by a counterflow heat exchanger. Preferably the tubesthemselves are closely thermally connected across their entire length,such that they can be used for exchanging heat during the isochorouspart of the Stirling cycle.

A favourable embodiment according to another aspect of the invention ischaracterized in that the first system of coupled pistons comprises alarge and a small piston, which can move in a first assembly of a largeand a small cylinder and that the second system of coupled pistonscomprises a large and a small piston, which can move in a secondassembly of a large and a small cylinder. In this embodiment the ratiobetween the diameters is according to the invention at leastsubstantially determined by the desired temperature difference betweenthe heat source and the cold source.

A favourable embodiment according to still another aspect of theinvention is characterized in that the four cylinders are provided witheight connections and that a system of valves is provided for mutuallyconnecting the eight connections for executing a Stirling cycle. In thisway a switchover can be made at the right moment, that means the mostoptimal moment from one part of the Stirling cycle to the next part.

The invention will now be further explained with a reference to thefigures, in which:

FIG. 1 represents a possible PV diagram of a Stirling cycle;

FIG. 2 schematically represents a Stirling motor or a heat pumpaccording to the invention, during a down-going movement of the pistons;

FIG. 3 schematically represents a Stirling motor or a heat pumpaccording to the invention, during an up-going movement of the pistons;

FIG. 4 schematically shows a hydraulic interconnection between thepistons.

FIG. 1 represents a possible PV diagram of a Stirling cycle, in which avolume of gas experiences an isothermal compression in a trajectory 1,next an isochorous heating in trajectory 2, next an isothermal expansionin trajectory 3 and finally an isochorous cooling in trajectory 4. In aStirling motor according to the state of the art, the four trajectoriesare continuously passed through in a chronological order, while in aStirling motor according to the invention all four trajectories arepassed through simultaneously in a continuous manner.

FIG. 2 schematically represents a Stirling motor or a heat pumpaccording to the invention, during a down-going movement of the pistons5, 6, 7, 8 in cylinders 9, 10, 11, 12. Cylinders 9, 10, 11, 12 have beenfilled with a gas, which is selected such that, within a predefineddetermined temperature range, a large amount external work can beexecuted. For low temperatures, helium for example can be taken, whilefor higher temperatures for example R-12 and R-22 cooling fluids may betaken. In an up-going or down-going movement, the gas is transported,during which it must pass a number of double slide valves 13, 14, 15,16.

Cylinders 9, 10 and slide valves 13, 14 constitute, together with theconnecting lines, the hot motor part of the Stirling motor. To this partheat is supplied continuously, such that a temperature, T_(high) ismaintained. Cylinders 11, 12 and slide valves 15, 16 constitute,together with the connecting lines, the cold motor part of the Stirlingmotor. From this part heat is removed continuously, such that atemperature T_(low), is maintained. Lines 17, 18 connect the hot motorpart with the cold motor part; together they constitute a counterflowheat exchanger and for that purpose they are thoroughly interconnectedby a bridge 19 with a very low heat resistance. For that purpose theymay be made for example of copper and be soldered together over theirentire length with the aid of silver solder.

Cylinders 9, 12 preferably have the same dimensions and cylinders 10, 11preferably have also the same dimensions. Moreover it can easily bederived that preferably the ratio between the areas of piston 5 andpiston 6 and of piston 8 and piston 7 should be taken equal toT_(high)/T_(low).

With the slide valves positioned such as shown in the figure, gas willbe pushed from the space underneath piston 6 to the space above piston 5and thereby expand, in the process of which its temperature will remainequal to the temperature of the hot motor part T_(high). Moreover, gaswill be pushed from underneath piston 8 to the space above piston 7, inthe process of which it will be compressed, while its temperature willremain the equal to the temperature of the cold motor part T_(low). Alsogas will be pushed from underneath piston 5, via line 17, to a spacewith the same volume above piston 8, in the process of which it willdeliver heat to a gas which is pushed from a space underneath piston 7,via a line 18 to a space with the same volume above piston 6.Summarising, during a down-going movement of the pistons all four thetrajectories of the Stirling cycle are passed through simultaneously.

FIG. 3 schematically represents a Stirling motor or a heat pumpaccording to the invention, during an up-going movement of the pistons.With the slide valves positioned such as shown in the figure, gas willbe pushed from the space above piston 6 to the space underneath piston 5and thereby expand, in the process of which its temperature will remainequal to the temperature of the hot motor part T_(high). Moreover, gaswill be pushed from above piston 8 to the space underneath piston 7, inthe process of which it will be compressed, while its temperature willremain equal to the temperature of the cold motor part T_(low). Also gaswill be pushed from above piston 5, via line 17, to a space with thesame volume underneath piston 8, in the process of which it will deliverheat to a gas which is pushed from a space above piston 7, via a line 18to a space with the same volume below piston 6. Summarizing, during adown-going movement of the pistons all four the trajectories of theStirling cycle are passed through simultaneously.

It must be noted that a rod 20, which couples the pistons 5, 6, 7, 8, isconnected to a flywheel in a manner well known in the art, and that arod 21, which couples the slide valves 13, 14, 15, 16, is controlled forexample by two cams on the flywheel, in such a manner that when thepistons 5, 6, 7, 8 have reached their lowest position, the slide valvesassume the position as shown in FIG. 3, while when the pistons 5, 6, 7,8 assume their highest position, the slide valves assume the position asshown in FIG. 2.

Instead of the slide valves, shown in FIG. 2 and FIG. 3, it is obviouslypossible to apply other types of valves, as long as they realize thefunctions as described with a reference to the figures. It may beadvantageous for example to use electrically operated valves and tocouple a position sensor or a speed sensor to rod 20. Instead of a rigidswitch timing, derived from the flywheel, it is possible then to use forexample a microprocessor to determine a more optimal switch timing,dependent upon the position and/or the speed of rod 20 and possibly uponT_(high) and T_(low).

With electrically operated valves the only rigid connection between thepistons in the hot motor part and the cold motor part is rod 20. FIG. 4schematically shows a possible embodiment of a hydraulic interconnectionbetween the pistons, which makes it possible to mount the cold motorpart and the hot motor part separately, in such a manner that the onlyconnections are the lines 17, 18 and a hydraulic interconnection 22. Rod20 is divided then in a part 20 a, connecting the pistons 5, 6 and apart 20 b, connecting the pistons 7, 8. Part 20 a is connected then to asmall piston 23 a and part 20 b with a small piston 23 b, which smallpistons can move inside their respective small cylinders 24 a, 24 b.Small cylinders 24 a, 24 b and hydraulic interconnection 22 are, asusual, filled with hydraulic oil.

The description as given with a reference to FIGS. 1-3, specificallyaddresses the Stirling motor. It is known in the art that a Stirlingengine also can be externally driven, in which manner a heat pump isobtained for which the description as given with a reference to FIGS.1-3 applies substantially unchanged. Also known is that the trajectoryof the PV diagram of FIG. 1 is passed through then the other way around.

1. A Stirling motor comprising: a hot motor part comprising, a firstpair of mutually coupled pistons, including a first large piston and afirst small piston which are movable in a reciprocal manner, and a firstassembly including a first large cylinder and a first small cylinder inwhich said first large piston and said first small piston respectivelymove, wherein said first assembly is configured to allow fluid to beexchanged between the first small cylinder and the first large cylinder;a cold motor part comprising, a second pair of mutually coupled pistons,including a second large piston and a second small piston which aremovable in a reciprocal manner, and a second assembly including a secondlarge cylinder and a second small cylinder in which said second largepiston and said second small piston respectively move, wherein saidsecond assembly is configured to allow fluid to be exchanged between thesecond small cylinder and the second large cylinder; a shaft or ahydraulic interconnection connecting the hot motor part and the coldmotor part; and a counterflow heat exchanger coupled to said hot andcold motor parts by two tubes so as to mutually thermally interconnectthe first large cylinder in said hot motor part with the second largecylinder in said cold motor part, and the second small cylinder in saidhot motor part to the second small cylinder in said cold motor part. 2.The Stirling motor according to claim 1, wherein a ratio between thediameters of the first large piston and the first small piston and aratio between the diameters of the second large piston and the secondsmall piston substantially equal a ratio between an operationaltemperature of the hot motor part and an operational temperature of thecold motor part.
 3. A Stirling motor according to claim 2, furthercomprising: eight connections among the first large cylinder, the firstsmall cylinder, the second large cylinder and the second small cylinder;and a system of valves mutually connecting the eight connections forexecuting a Stirling cycle.
 4. A heat pump comprising: a hot pump partcomprising, a first pair of mutually directly coupled pistons, includinga first large piston and a first small piston which are movable in areciprocal manner, and a first assembly including a first large cylinderand a first small cylinder in which said first large piston and saidfirst small piston respectively move, wherein said first assembly isconfigured to allow fluid to be exchanged between the first smallcylinder and the first large cylinder; a cold pump part comprising, asecond pair of mutually directly coupled pistons, including a secondlarge piston and a second small piston which are movable in a reciprocalmanner, and a second assembly including a second large cylinder and asecond small cylinder in which said second large piston and said secondsmall piston respectively move, wherein said second assembly isconfigured to allow fluid to be exchanged between the second smallcylinder and the second large cylinder; a shaft or a hydraulicinterconnection connecting the hot pump part and the cold pump part; anda counterflow heat exchanger coupled to said hot and cold pump parts bytwo tubes so as to mutually thermally interconnect the first largecylinder in said hot pump part with the second large cylinder in saidcold pump part, and the second small cylinder in said hot pump part tothe second small cylinder in said cold pump part.
 5. The Stirling pumpaccording to claim 4, wherein a ratio between the diameters of the firstlarge piston and the first small piston and a ratio between thediameters of the second large piston and the second small pistonsubstantially equals a ratio between an operational temperature of thehot pump part and an operational temperature of the cold pump part.
 6. AStirling pump according to claim 5, further comprising: eightconnections among the first large cylinder, the first small cylinder,the second large cylinder and the second small cylinder; and a system ofvalves mutually connecting the eight connections for executing aStirling cycle.